The invention relates to a passively mode-locked short pulse laser arrangement comprising a laser resonator to which a pump beam is supplied, a laser crystal, in particular a titanium-sapphire-(Ti:S-)laser crystal, and laser mirrors, the laser crystal, which is subjected to a thermal load on account of the beam focussing, being mounted on a cooling body provided for the removal of heat, which cooling body includes a bore for the passage of the laser beam.
Such laser arrangements are used for scientific purposes, on the one hand, and can be used in material processing, on the other hand, particularly if fine structures are to be produced.
In the mode-locked state, a laser emits laser pulses instead of a continuous laser light (continuous wave (cw) operation), by storing energy and emitting it thereafter in pulse-like manner. The duration of the periods of these pulses will generally correspond to the round trip time of the pulses in the laser resonator, and, e.g., with a length of the linear resonator of 2 m, pulses with a frequency of approximately 75 MHz will be generated; here, the laser light pulse passes the laser resonator in both directions, which in the instant example will correspond to a length of 4 m. For mode-locking, a loss is periodically introduced (with the resonator round trip frequency)xe2x80x94e.g. by deflecting or blocking the laser beamxe2x80x94so that the laser begins to pulse. This results in a peak power of the pulses that is substantially higher (amounting to 100 kW to 200 kW, e.g.) than the output power of the laser in cw operation (which is 150 mW to 300 mW, e.g.).
Basically, it can be differentiated between two types of mode-locking.
In active mode-locking, a periodic loss is introduced by means of an active element, a modulator, which is supplied with energy from the outside via a driver, e.g. by the modulator periodically deflecting the laser beam from its direction of propagation. Thus, the laser is forced to perform its laser activity in those time intervals in which there is a lower loss, whereas the laser can store energy in those time intervals in which there are high losses.
In passive mode-locking, the effect of an optical non-linearity in the resonator is utilized, i.e. an optically non-linear element is arranged in the path of the laser beam, and this non-linear element changes its optical properties, such as the transmission or reflectivity, proportionally to the intensity of the laser beam. As such a non-linear element, the laser crystal itself may, e.g., be used which forms a so-called saturatable absorber in which the loss will become the lower the higher the intensity of the impacting laser light. By a fluctuation in the laser power, a pulse is generated which xe2x80x9cseesxe2x80x9d a substantially lower loss than does the laser in cw operation (cf. also U.S. Pat. No. 5,079,772 A). The laser body (solid state laser) consists of a non-linear material whose optical xe2x80x9cthicknessxe2x80x9d varies with the field intensity distribution of the laser radiation. The non-linear index of refraction, e.g., is a function of the square of the field intensity, i.e. the laser beam whose field intensity distribution may be considered to be like a Gaussian curve, effectively xe2x80x9cseesxe2x80x9d an element with an optical thickness that varies over its cross-section in the case of a laser crystal having plane-parallel faces. In this manner, a focussing lense results from a plane-parallel non-linearity.
This optical Kerr effect may be utilized for mode-locking in two manners (so-called xe2x80x9cKerr-lens mode-lockingxe2x80x9d): In the case of the so-called xe2x80x9csoft aperturexe2x80x9d (cf. Spence et al., Optics Letters, Jan. 1, 1991, Vol. 16, p. 42-44), the pump beam (in Ti:S lasers the energy is supplied by means of green laser, such as, e.g., argon laser) is very much focussed in the laser crystal so that the resonator beam produced by the Tixe2x80x94S laser (approximately 800 nm, infrared) may then take up the greatest part of the pump energy, i.e. may have the highest gain, if it has the smallest diameter. Thus, the higher the intensity, or the field strength, respectively, of the pulse, the more the laser pulse will be focussed and the greater its gain at any passage through the laser crystal, whereby its intensity is increased again. This positive feedback leads to a stable mode-locking.
In the case of the so-called xe2x80x9chard aperturexe2x80x9d (cf. e.g. U.S. Pat. No. 5,079,772 A) the effect is utilized that an aperture restricts the resonator beam at a site where it has a larger diameter at that time when the intensity (field strength) is lower, and has a smaller diameter at that time when the intensity is higher and the resonator beam thus is focussed in the laser crystal.
Other passive mode-locking techniques, e.g. semiconductor-saturable absorbers, are also known, cf. e.g. R. Fluck et al., xe2x80x9cBroadband saturable absorber for 10-fs pulse generationxe2x80x9d, Optics Letters, May 15, 1996, Vol. 21, No. 10, pp. 743-745.
To generate extremely short and thus high intensity pulses (in the femtosecond range) it is necessary to control the group dispersion in the resonator. Since pulses which are extremely short in the time range have a broad spectrum in the frequency range, there occurs the undesirable effect that in the laser crystal, the different frequency components xe2x80x9cseexe2x80x9d a different index of refraction and thus a different optical length of the laser crystal and thus are differently delayed when passing through the laser crystal. Thus, the pulses are lengthened again. To counteract this, the beam can be partitioned in terms of frequency by arranging optical prisms; the different frequency components will travel paths of different lengths, and in a further prism the beam is collimated (directed in parallel) again. As a consequence, the different frequency components will be delayed just reversely as in the laser crystal, whereby the dispersion introduced in the laser crystal is compensated again (cf. U.S. Pat. No. 5,079,772 A).
According to a further suggestion (e.g. Stingl et al., xe2x80x9cGeneration of 11-fs pulses from a Ti:sapphire laser without the use of prismsxe2x80x9d, Optics Letters, Feb. 1, 1994, Vol. 19, No. 3, pp. 204-206), special laser mirrors are used which are assembled of many ( greater than 40) layers, the different components of the wave lengths penetrating to different depths in the mirror before being reflected. Accordingly, the different components of the wave lengths of the laser beam are delayed in the mirror for different periods of time; the short-wave components are reflected at the surface, whereas the long-wave components are reflected at a deeper location in the mirror and thus experience a delay as compared to the short-wave components. The advantage of the last-mentioned method is a better dispersion compensation, whereby extremely short pulses can be produced directly from a resonator.
Irrespective of the dispersion compensation technique used in detail, it is also important for the dispersion control, for producing extremely short laser pulses (in the order of 10 fs and therebelow), to keep low the material dispersionxe2x80x94primarily in the laser crystal, and for this it is suitable to use a thin, i.e. short, laser crystal (that is a laser crystal having a short path length). For reasons of compensation, the laser crystal should have a high dotation (e.g., already within 2 mm, it absorbs more than 70%). To keep the pumping threshold as low as possible and to thus ensure an efficient conversion of pumping power into laser output power, the pumping beam and the resonator beam should be focussed as much as possible. The greatly reduced dimensions of the pumped volume of the laser crystal will thus lead to an increased thermal load.
Thus, it is an object of the invention to provide a laser arrangement of the initially defined type in which an improved heat removal is provided for the laser crystal so that an increased thermal load on the laser crystalxe2x80x94at comparatively small dimensions of the samexe2x80x94and consequently, an increase in the output power will be rendered possible.
The inventive laser device of the initialy defined type thus is characterised in that a crystal mount of a material with good heat conducting properties is provided on the cooling body and that the laser crystal is held in an opening of this crystal mount, with lateral abutment on oppositely arranged walls of the opening of the crystal mount, the opening in the crystal mount being in alignment with the bore in the cooling body.
In contrast to continuous wave solid state lasers, in which crystals having a length of from 5 to 10 cm are used, and to conventional femtosecond solid state lasers which use a crystal having a length of from 10 to 20 mm and a cross-section of 4xc3x974 mm or larger, in this embodiment, laser crystals having a length of a few mm and an extremely small cross-section, in the order of 1 mm, e.g., can be used, whereby consequently also extremely short laser pulses (less than 10 fs) can be generated. This not only results in a reduction of costs of the laser crystal itself but, in combination with the above-indicated crystal mount which has a high thermal conductivity, it also enables an effective removal of heat from the laser crystal. In this connection it is also essential that by the small laser crystal dimensions, the path in the laser crystal for the heat removal from the pumped volume of the laser crystal lying in a middle zone (e.g., with a diameter of approximately 10 to 50 xcexcm) to the surfaces of the crystal mount is greatly shortened. The crystal mount in turn transmits the heat to the cooling body whose temperature is held at 10xc2x0 C., e.g. This temperature control on the cooling body may be coped with technically well (condensation problems may, however, occur if cooling is effected to below the temperature mentioned). Usually, the cooling body will be made of aluminum. By reducing the temperature in the interior of the crystal, the output power of the pulse laser can be improved. This, i.a., is a consequence of the fact that the lifetime of the electrons in the upper laser level decreases with an increasing temperature in the laser crystal. Tests using Ti:S laser crystals have shown that when using the crystal mount provided according to the invention, an output increase of up to 20% as compared to earlier embodiments using a cooling body could be achieved. From the nature of passive mode locking it follows that on account of the increased output power, further pulse shortening can be attained at equal pumping power.
To make adjusting the laser crystal in the crystal mount as easy as possible, it is particularly advantageous if thexe2x80x94e.g. platelet-shapedxe2x80x94crystal mount has a slit-shaped opening. In this embodiment, the laser crystal can be exactly positioned at the desired site within the slit-shaped opening. To allow also for an insertion and positioning of the laser crystal from the rim of the crystal mount, it is furthermore suitable if the slit-shaped opening extends from a rim of the crystal mount into the same. In this embodiment, the laser beam may be arranged also at or close to the rim of the crystal mount. Furthermore, for tightly clamping the laser crystal in the opening without special additional means, it has proven particularly advantageous if the slit-shaped opening extends as far as to shortly in front of the opposite rim of the platelet-shaped crystal mount, and if the material of the crystal mount that remains forms a link of the type of a (film) hinge, the two halves of the crystal mount which are separated from each other by the slit-shaped opening forming legs which are pivotable relative to each other. These legs tightly clamp the laser crystal between them. To avoid as far as possible an undesired tearing of the web between the legs, if the legs are to be straddled during insertion of the laser crystal, it is also advantageous if the insertion-type slit-shaped opening ends in a widened round.
To apply the clamping force, a pressure force as such could be applied onto the two legs from the outside, e.g. by a type of cramp or the like device. A particularly simple form of force application is, however, possible if the legs have transverse bores extending in a direction transverse to the slit-shaped opening for accommodating a bracing element which pivots the legs relative to each other. In this connection it is a particularly advantageous further development that the transverse bore in one leg is an overdimensioned smooth through-bore and the transverse bore in the other leg is provided with an internal thread and that the bracing element is a straining screw whose shaft freely extends through the smooth through-bore of the one leg and is screwed into the threaded bore of the other leg. In this embodiment, thus simply a tightening or clamping screw is provided, by the rotation of which the two legs can be braced or straddled so as to tightly clamp or release the laser crystal between them.
To provide for a better access to the laser crystal held in the crystal mount, for cleaning purposes, it has also proven suitable if the opening of the crystal mount has a chamfered rim at its side facing away from the cooling body, the laser crystal extending as far as to the chamfer.
As has already been mentioned, the embodiment according to the invention can particularly be used in combination with comparatively small laser crystals, and thus it is particularly advantageous if the laser crystal has the form of a parallelepiped having thickness dimensions in the order of approximately 1 mm and a length of approximately 2 mm, the diameter of the pumped volume being in the order of 10 xcexcm.